This invention pertains to turbines, more specifically to the testing of turbine control systems, such as overspeed prevention systems.
Turbine machines, especially steam turbines used in commercial power plants, generally employ control systems that perform a variety of functions, including tripping. xe2x80x9cTrippingxe2x80x9d is the shutting down of a turbine when certain abnormal situations occur, for example, low bearing oil pressure, high bearing temperature, and rotor overspeed. Rotor overspeed, if unchecked, could cause a rotor to fly apart, resulting in substantial damage, and in some instances, catastrophic results. Consequently, most steam turbines are equipped with either electric or hydraulic control systems or both, and backup mechanical overspeed trip devices to prevent rotor overspeed. These devices must be periodically tested to ensure proper functioning. In most instances, testing turbine overspeed trip systems requires driving the turbine rotor to trip set-points, typically set at 103-120% of the normal design speed. See, e.g., U.S. Pat. Nos. 5,133,189 and 5,292,225.
In nuclear power plants, small to medium power turbines are routinely used as prime movers (source of rotation), and are periodically tested to ensure proper functioning. Generally, less risk is involved when overspeed trip testing is performed at a time when the turbine is not required to be operational, for example, during refueling outages. During refueling outages, maintenance and testing activities which, if delayed, would delay the return to service (productivity) of the power plant are identified as being xe2x80x9con critical-path.xe2x80x9d By contrast, maintenance and testing activities performed during an outage that do not increase the outage duration are identified as xe2x80x9coff critical-path.xe2x80x9d Nuclear power plant management typically prefers that all maintenance and testing activities, including overspeed testing, be performed off critical-path where possible. However, the costs associated with conducting these tests can be significant because an alternate source of steam has typically been required to spin the turbine since the reactor can no longer produce steam. These costs can include the rental of an alternative steam source capable of spinning the turbine rotor beyond its normal trip set-points, in addition to manpower costs for engineering, maintenance, and operations support. Furthermore, the logistics of installation, operation, and removal of the required equipment can add complexity to an already complex refueling outage schedule.
Alternatively, overspeed trip testing could be conducted using steam provided by the reactor once it is again operational. However, this testing method is generally not preferred because of the losses in productivity that result due to the delay in return to service. More specifically, when testing a turbine using steam provided by the reactor, the tests are performed during the plant start-up from the refueling outage. This testing method is generally considered xe2x80x9con critical-pathxe2x80x9d because the testing activity becomes a series activity in the start-up sequence.
To rotate a turbine rotor beyond normal trip set-points requires high power motive drive systems, capable of overcoming windage effects. xe2x80x9cWindagexe2x80x9d generally refers to a loss due to fluid drag on a rotating body. Windage losses are a function of the speed of a turbine rotor. Windage effects for rotors spinning in air at high speeds are significant.
An unfilled need exists for a device to test turbine control systems that reduces the power requirement needed to spin the turbine rotor beyond its normal trip set-points during overspeed testing, and that allows overspeed testing to be performed off critical-path. This device should allow testing without subjecting the tested turbine to unacceptable stresses, such as near-sonic velocity at the turbine rotor tips.
I have discovered a reliable and inexpensive device and method for testing turbine control systems. The device may be adapted to test most turbine rotors. The turbine control testing device comprises an operator control system, a drive motive power assembly, and a purge gas assembly. Once installed, the testing device is used to accelerate the turbine rotor to its test velocity without the use of steam. Rotor speed and acceleration may be controlled with relatively high precision. This minimizes the likelihood that the turbine will be damaged as a result of sonic velocity or other mechanical failure. The purge gas assembly provides a purge gas whose sonic velocity is higher than that of air, thereby reducing sonic velocity risks. Windage losses and power requirements are both minimized by selecting a purge gas with a low atomic/molecular weight.